Prem Pallav

Occlusal wear simulation with the ACTA wear machine

This paper describes how the clinical conditions in stress bearing areas are thought to be simulated in the ACTA wear machine. The wear types simulated are erosive wear and contact sliding wear in the presence of a third-body medium, consisting of natural food substances. Wear due to surface fatigue may also be studied with the wear machine. Wear rates for a wide range of resin composites, an amalgam and a glass polyalkenoate (ionomer) cement obtained with the ACTA wear machine correlated with an average correlation coefficient of 0.90 with data collected from clinical trials. The results justify the experimental set-up as being one that includes a realistic simulation of the complexity of clinical wear.

Annealing as a mechanism of increasing wear resistance of composites.

The objective of this study was to examine the influence of a short-term exposure to heat (125 degrees C) on the wear resistance of composites. Both light- and chemically initiated materials improved by 20-60%. However, the improvement was also attained in the course of time if the materials had not been exposed to heat. The heat-induced improvement could not be explained by a continuation of polymerization but rather by stress relief, which is common for annealing processes. Polymerization shrinkage stresses, initially concentrated mainly around the filler particles, became more homogeneously distributed by the heat treatment. The long-term improvement of the non-heat-treated materials was based on the same mechanism, but proceeded more gradually.

The Influence of Admixing Microfiller to Small-particle Composite Resin on Wear, Tensile Strength, Hardness, and Surface Roughness

A series of photo-initiated composite resins, with a Bis-GMA-TEGDMA resin matrix, was investigated in vitro for occlusal wear, surface roughness, diametral tensile strength, and hardness. The materials contained mixtures of ground quartz (macro filler, 3 μm) and colloidal silica (micro filler, 0. 04μm), with total filler being constant at 68.5% by volume. The study was conducted to determine the influence of an increasing ratio of micro to macrofiller in the mixtures. When the macrofiller was gradually replaced by 3, 5, 7.5, 10, 15, and 20 vol% of microfiller, wear resistance increased substantially, ultimately to a constant level. Any influence on surface roughness or tensile strength and hardness could not be demonstrated.

Influence of Shearing Action of Food on Contact Stress and Subsequent Wear of Stress-bearing Composites

The influence of sliding action of the antagonist on occlusal three-body wear of composites and an amalgam was investigated in vitro by gradual change in the distance between the opposing substrates. When the distance was decreased from 10 μm to approximately 3 μm, wear increased significantly by a factor of two to three and was exclusively of erosive nature. At a slurry-film thickness of approximately 1 μm, direct contacts between the antagonist and protruding composite filler particles started to occur. This consequently slowed the erosive wear. Ultimately, direct contact phenomena predominated, decreasing the wear rate of the various materials to different degrees. Loss of material due to subsurface fatigue could not be demonstrated with a contact pressure of 45 MPa at which the experiments were performed. From this study, it can be concluded that minor alterations of the food-film thickness at the contact areas result in considerable changes in wear rates and wear-rate ranking of composite materials, which may partly explain inconsistencies among clinical trials.

Wear rates of composites, an amalgam, and enamel under stress-bearing conditions

This article describes an in vitro technique that, within in a few days, can predict the long-term occlusal wear of composites. The laboratory data of our study and the clinical observations of various authors correlate well. For 19 different products, the wear relative to an amalgam under stress-bearing conditions is given within statistically justified boundaries.

Influence of polymerization mode and C-factor on cohesive strength of dual-cured resin cements.

OBJECTIVES The aim of this study is to determine the influence of the C-factor and the mode of polymerization on the cohesive strength of various dual-cure resin cements. METHODS Three curing conditions were tested; chemical curing with free shrinkage conditions (C=0), and constraint shrinkage conditions (C=25), and dual-curing with free shrinkage conditions (C=0). Opaque polyethylene, brass (pretreated with Clearfil SE bond), and transparent polyethylene tubes respectively, were filled with the different cements. The tubes were 20mm long with an inner diameter of 1.6 or 1.8mm. Five cements, DC Core Automix, Panavia F 2.0, Maxcem, Multilink, and RelyX Unicem, were tested with ten specimens per group. The specimens were trimmed to an hour-glass shape with a neck diameter of 1mm, stored in water (37°C, 24h), and subjected to microtensile testing (1mmmin(-1)). SEM analysis was carried out on chemically cured samples of DC Core Automix C=0 and C=25. Data were statistically analyzed (Two-way ANOVA, Tukeys post hoc test, p<0.05). RESULTS Most cements showed no significant differences between the curing modes. A high C-factor negatively influences the cohesive strength of some cements. SEM analysis shows that chemical curing of DC Core Automix in a high C-factor environment leads to more and larger microvoids in the cement. SIGNIFICANCE Constraint shrinkage conditions, i.e. a high C-factor, can negatively influence the physical properties of a dual-cured resin cement, which would clinically be the case in the confined space of a root canal or post space preparation.

Shear bond strength of three dual-cured resin cements to dentin analyzed by finite element analysis.

OBJECTIVES To determine the shear bond strength to bovine dentin of dual-cured resin cements cured in different circumstances, the contraction stress and volumetric shrinkage in both polymerization modes, and to review the failure stress distribution at the cement-tooth interface with finite element analysis. METHODS The volumetric shrinkage of RelyX Unicem, Panavia F 2.0 and DC Core Automix was determined by mercury dilatometry. Polymerization contraction stress was determined using a constraint tensilometer set-up. For the shear bond strength test, cement discs on bovine root dentin (self-cured and dual-cured), composite discs cemented to dentin (self-cured and dual-cured), and dentin cemented to dentin (self-cured) specimens were fabricated. Specimens were stored in water for 24h (37°C, 100% humidity) and tested (crosshead speed 1mmmin(-1)). FE modeling of the specimens was carried out in order to calculate the maximum shear stresses in the cement-dentin interface. Differences between groups were determined using two-way ANOVA with Tukey post hoc tests, and paired samples t-tests (α<0.05). RESULTS Panavia F2.0 showed significantly lower volumetric shrinkage than the other cements. Dual-curing lead to higher contraction stresses for all tested cements compared to self-curing. RelyX Unicem showed higher volumetric shrinkage when dual-cured. Shear bond strength and maximum shear stress was positively influenced by dual-curing. DC Core Automix performed best and Panavia F2.0 worst in terms of shear bond strength and maximum shear stress. SIGNIFICANCE Curing mode may play an important role in the final bond strength to dentin of indirect restorations, depending on the material used.

Effect of long-term repeated deflections on fatigue of preloaded superelastic nickel-titanium archwires

INTRODUCTION The aim of this in-vitro study was to investigate the changes in force delivery of superelastic nickel-titanium archwires used in combination with a self-ligating bracket system after dynamic fatigue-loading in a 3-bracket model under controlled temperature. METHODS Samples of 2 superelastic nickel-titanium (active austenitic) wires, a conventional nickel-titanium wire, and a stainless steel wire, all 0.014-in round, were divided into 2 groups: static deflection and dynamic deflection. The static specimens were under a constant deflection of 3.0 mm. The dynamic specimens had the same constant deflection of 3.0 mm but were subjected to additional repeated deflections of 0.5 mm, applied by a fatigue tester. The test situation simulates a patients archwire under deflection and subjected to occlusal contact during 1, 100, 10,000, and 100,000 cycles. Fatigue changes were assessed with a 3-point bending test. RESULTS Type of wire, loading or unloading, and number of cycles as within-subject factors were statistically significantly different. No statistically significant difference between the test condition, static vs dynamic, was found. The repeated deflections of 0.5 mm were not enough to induce an extra effect of fatigue. CONCLUSIONS Occlusal forces transferred to a considerably deflected archwire, such as in the large malalignments in the early stages of orthodontic treatment, will have no fatigue effect on the unloading force of that archwire.